100-400 I vs II for Airshows and motorsports

Hi,

I have been using the 100-400 I for airshows for several years now and have been generally very happy with the lens, but with the vII out I can't help but wonder if I should upgrade. The improved spec that I am most interested in is the IS. As you know for shooting prop planes you need to pan at slow shutter speeds; ideally around 1/60s. Now panning definitely requires good technique and a lot of practice, but in my experience IS in mode 2 can't hurt.

So the question is for anyone that has used both the vI and vII for airshows, motorsports, or some other type of photography that requires panning at relatively slow shutter speeds, do you see an improvement in the vII over the vI? Do you see more keepers?

Thanks!